1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunction machine including at least two of these functions, and an image density control method performed by the image forming apparatus, and more particularly, to an image forming apparatus and an image density control method using a two-component developer.
2. Discussion of the Background Art
In general, an electrophotographic image forming apparatus such as a copier, a printer, a facsimile machine, etc., forms an electrostatic latent image on an image carrier, develops the electrostatic latent image with a toner, and then transfers the toner image onto a sheet of recording medium.
As methods of developing electrostatic latent images, methods using two-component developer including toner and magnetic carrier are widely known. In such two-component developing methods, the two-component developer, which is hereinafter simply referred to as a developer, is deposited on the image carrier by magnetism of magnetic poles provided inside the image carrier so as to form a magnetic brush thereon. The magnetic brush is rubbed on the electrostatic latent image to develop the electrostatic latent image. At present, such a two-component developing method is widely used because it facilitates color image forming.
However, in such two-component developing methods, if a toner concentration, indicated by a ratio such as weight ratio of toner to carrier in the developer, is excessively high, toner might scatter on a background of an output image or detail resolution capability might be degraded. By contrast, when the toner concentration is excessively low, an image density of a solid image portion might decrease or magnetic carrier might be deposited on the image carrier. Therefore, it is important to keep toner concentration in the developer within a preferable range, for example, by controlling toner supply based on detection result of the toner concentration in the developing device.
However, even if the toner concentration is kept constant, image density can fluctuate during continuous printing depending on the image area ratios of output sheets. For example, when an image of higher image area ratio is output, toner consumption is greater and more toner is newly supplied to the developing device compared to a case in which the image area ratio of an output sheet is lower. Hereinafter, toner replacement amount refers to a ratio of newly supplied toner to existing toner in the developing device. Newly supplied toner is not sufficiently charged, and therefore it is difficult to raise an average toner charge to a preferable level before a subsequent image formation when the toner replacement amount is larger, resulting in a rise in image density. By contrast, when an image of lower image area ratio is output, toner consumption is smaller and less toner is supplied. That is, the amount of toner that remains in the developing device for a relatively long time period is larger compared to a case in which the image area ratio of an output sheet is higher. The longer toner remains in the developing device, the more that toner is agitated and excessively charged, causing an increase in the average charge and a decrease in image density.
In a prior application, an image forming apparatus that includes an information detection means for detecting information to determine the toner replacement amount, and a correction means has been proposed. The toner replacement amount can be determined, for example, by the image area ratio of the output sheets. By using the toner replacement amount determined based on the detection result of the information detection means, the average toner charge for a subsequent image formation in continuous image forming is detected, and then the correction means corrects a reference value for controlling the toner concentration. Thus, image density is kept at a constant density by controlling the toner concentration in the developing device.
However, charge characteristics of the toner tend to change over time and/or with changes in environmental conditions, making consistent toner concentration control difficult.